1. Field of the Invention
The invention relates to a process for preparing substituted alkylsilanes by addition of silanes having an Si—H bond onto unsaturated aliphatics in the presence of a specific Ir catalyst/cocatalyst system.
2. Description of the Related Art
Substituted alkylsilanes are of tremendous economic interest for many fields. They are used, for example, as bonding agents or as crosslinkers.
The platinum- or rhodium-catalyzed hydrosilylation of unsaturated compounds has already been examined many times. The use of platinum-containing hydrosilylation catalysts is described, for example, in U.S. Pat. Nos. 2,823,218 A and 3,159,601. U.S. Pat. Nos. 3,296,291 A and 3,564,266 A mention the use of rhodium catalysts. The product yields are often very low at 20-45%, which can be attributed to considerable secondary reactions.
According to U.S. Pat. No. 4,658,050 A, JP 6100572 A and EP 0709392 A, iridium catalysts are used in the hydrosilylation of allyl compounds by alkoxy-substituted silanes. The Japanese patent JP 07126271 A is concerned with the hydrosilylation of allyl halides by chlorodimethylsilane. Disadvantages of these processes are either moderate yields, an uneconomically high catalyst concentration and/or a very short catalyst life. DE 10053037 C and EP 1156052 A describe processes in which low molecular weight, preferably cyclic dienes are added as cocatalysts in order to reduce the amounts of catalyst required. However, these cocatalysts have the disadvantage that they also react with the corresponding silanes and these reaction products are difficult to separate off from the desired target products by distillation.
A further disadvantage of these systems is that the reaction bottoms after conclusion of the reaction are contaminated with noble metal residues and a further isolation/concentration of the residues by extraction or precipitation cannot be carried out commercially, so that the reaction bottoms have to be worked up in their entirety.